P-wave Velocity Measurements Research Articles

Small-strain thermo-mechanical performance of lunar mare and highlands regolith simulants under Earth's atmospheric pressure and in vacuum

Geotechnical Lunar regolith simulants are developed so that engineers and researchers can explore regolith-equipment interactions ahead of deployment on the Lunar surface. A good understanding of thermo-mechanical interactions of Lunar regolith is critical to ensure the success of exploratory missions. Simulants are often evaluated based on their similarity to Lunar regolith in terms of grain-size distribution, particle shape, specific gravity, mineralogy, and chemical composition. However, the mechanical behavior of a simulant is also affected by its porosity, pore constituents, and the stress state. In this paper, the small-strain thermal and mechanical responses of two well-known commercial Lunar regolith simulants (Mare and Highland) are assessed as a function of porosity at Earth's atmospheric pressure and under vacuum. Measurements of thermal conductivity and p-wave velocity are used to monitor the impact of changes in porosity (i.e., density) and chamber pressure. The p-wave velocity data for the two simulants collapses into a single linear trendline when plotted against simulant porosity while the thermal conductivity data collapses into a single linear trendline when plotted against bulk density. Thermal conductivity is also strongly affected by chamber pressure. Thermal boundary effects make measurements conducted under 10−1 Torr inconclusive. Altogether, the results of the study provide a benchmark against which the small-strain mechanical response of other simulant candidates can be compared.

Effects of overconsolidation on seismic rock physics — A comparative study between unconsolidated sands and weakly cemented sandstone

In seismic rock physics, models typically assume rocks are normally consolidated, often neglecting the effects of overconsolidation. Recent studies reveal that overconsolidated sands, resulting from stress release, show different physical property variations than normally consolidated sands. However, similar research is lacking for weakly cemented sandstones, a common reservoir analog. This study aims to systematically compare the effects of overconsolidation and stress direction on unconsolidated sandstones and weakly cemented sandstones. It also assesses the impact of overconsolidation on seismic interpretation, monitoring, and rock physics modeling applied to the two types of rocks. Experimental measurements of porosity, P-wave, and S-wave velocities were collected under different stress paths between loading and unloading. For weakly cemented sandstone, well-log data from the Norwegian Sea and Barents Sea were compared with experimental data. Analyses focused on the velocity-porosity relationship, VP/VS ratio vs. AI, and VP/VS ratio vs. effective stress.

Unsupervised weathering identification of grottoes sandstone via statistical features of acoustic emission signals and graph neural network

Weathering features of sandstone heritage can be recognized by using artificial intelligence (AI) based surrogate models, and most models perform classification tasks for types based on precise labels. But there are lack of prior validated knowledge of the weathering or untagged historical data for complex weathering conditions in many cases. To this aim, a unsupervised graph neural network (GNN) based on the statistical features of the acoustic emission (AE) signals is constructed. Firstly, taking unweathered sandstone as a reference, we define 4 weathering levels of sandstone ranging from I to IV based on pore indicators. We selected 11 statistical features that are high correlated with pore of sandstone. Then, this GNN is constructed and trained by 2880 sets of statistical measured AE signals. Compared with AEs, LOF and IF models, GNN achieves the best identification performance among the four evaluation criteria. Each iteration of the GNN network is fitting the feature information of the signals and their neighbors. By data dimensionality reduction techniques, when the GNN stops iterating, it will be easy to distinguish unweathered AE signals from weathered one by comparing the reconstruction error of each signal. Furthermore, when the nearest neighbor’s k gradually increases, the AUC of GNN also gradually increases and then tend to stable when k equals to 50–100. While the hidden layers of the network aggregates less information about the neighborhood features of the signals and cannot distinguish significantly between unweathered and weathered signals when the value of k is small. As the depth of the network deepens, the feature values between signals become more and more similar, their reconstruction errors in the output layer of the network to become more similar, making it difficult to distinguish unweathered AE signals from weathered AE signals via GNN. Meanwhile, GNN adopts more AE features and considers the similarity between each features. This can greatly eliminate various errors caused by wave velocity measurement, greatly improving the robustness of AE detection. Hence, the GNN model presented addresses the limitations of relying solely on P-wave velocity measurements to assess the degree of sandstone weathering at stone cultural heritage.

Quantification of the Uncertainty in Ultrasonic Wave Speed in Concrete: Application to Temperature Monitoring with Embedded Transducers.

This article presents an overall examination of how small temperature fluctuations affect P-wave velocity (Vp) measurements and their uncertainties in concrete using embedded piezoelectric transducers. This study highlights the fabrication of custom transducers tailored for long-term concrete monitoring. Accurate and reliable estimation of ultrasonic wave velocities is challenging, since they can be impacted by multiple experimental and environmental factors. In this work, a reliable methodology incorporating correction models is introduced for the quantification of uncertainties in ultrasonic absolute and relative velocity measurements. The study identifies significant influence quantities and suggests uncertainty estimation laws, enhancing measurement accuracy. Determining the onset time of the signal is very time-consuming if the onset is picked manually. After testing various methods to pinpoint the onset time, we selected the Akaike Information Criterion (AIC) due to its ability to produce sufficiently reliable results. Then, signal correlation was used to determine the influence of temperature (20 °C to 40 °C) on Vp in different concrete samples. This technique proved effective in evaluating velocity changes, revealing a persistent velocity decrease with temperature increases for various concrete compositions. The study demonstrated the capability of ultrasonic measurements to detect small variations in the state of concrete under the influence of environmental variables like temperature, underlining the importance of incorporating all influencing factors.

Effect of grain size on ductility and failure mechanism of fiber-reinforced coral sand cement-based composites

Fiber offers the potential for enhancing the resistance to deformation in engineering cement-based composites (ECC). However, replacing quartz sand in ECC with coral sand containing a large number of internal pores will make the interaction between aggregate, interface transition zone, and cementitious materials more complex. To investigate the influence of coral sand particle size and Polyvinyl alcohol (PVA) fiber content on the compression behaviour and internal microstructure of fiber-reinforced coral sand cement-based composites (FRCSCC), we conducted uniaxial compressive strength tests and real-time measurements of P-wave and S-wave velocities. We tested various mechanical parameters, such as density, elastic modulus, compressive strength, wave velocities, and toughness index. The findings revealed a notable trend in relation to the coral sand particle size. Specifically, as the particle size increased, the compressive strength of FRCSCC decreased by 26 % and the elastic modulus reduced by 11 %. During uniaxial compression, the calculated cumulative voltage energy (CVE) from P-wave and S-wave can be divided into three stages: elastic deformation, microcrack and macrocrack propagation, and main crack formation. Moreover, the increase of coral sand particle size from 0.075 mm to 1.00 mm resulted in the rise of residual stress from 8.7 % to 18.2 %, followed by the increase of toughness index from 2.5 to 3.2. Scanning electron microscope (SEM) images reveal that the main failure modes of PVA fiber in FRCSCC specimens during uniaxial compression are debonding pull-out and tearing. Larger particle sizes of coral sand are associated with uneven fiber dispersion, increased fiber fractures, and diminished ductility of the specimens. The results establish a theoretical foundation for offshore engineering construction.

Characterization of Corrosion-Induced Fracture in Reinforced Concrete Beams Using Electrical Potential, Ultrasound and Low-Frequency Vibration

The paper deals with the non-destructive experimental testing of the reinforced concrete beams under progressive corrosion. A series of experiments using electrical potential, ultrasound and low-frequency vibrations techniques are reported. Electrical potential and natural frequencies were used to characterise and monitor the corrosion process at its initial state. The P-wave velocity measurements were proved to be effective in quantitative assessment of the level of corrosion as it progresses. The possibility of early detection of damage using a proposed damage index and diagnostic framework is promising for possible applications in the non-invasive diagnostics of reinforced concrete elements.

Architecture of interacting deformation band fault zones in siliciclastic rocks and its influence on geomechanical property distribution

Spatial organization of strain-hardening zones and the adjacent damage zones associated with faulting of porous siliciclastic rocks influence the across-strike variation of geomechanical properties. The across-fault geomechanical property distribution is expected to change when fault zones of distinct orientations interact. We performed outcrop-scale structural analysis along scanlines trending perpendicular to fault strike to constrain the extent of fault zones oriented dominantly along NE-SW and NW-SE. Young's Modulus (E) and P-wave velocity (Vp) measurements on deformation bands (DBs) and host rocks along such scanlines are further utilized to better characterize the fault zones. The selected outcrop exposes Cretaceous siliciclastic rocks in Rio do Peixe Basin, NE Brazil. In this study, Vp measurements coupled with detailed structural investigation have been integrated with grain size analysis for the first time. We observe that DBs oriented parallel to the major fault sets show grain size reduction along with higher E and Vp relative to the adjacent host rocks, indicating cataclasis and strain-hardening. The expected variation of geomechanical properties across the relatively late-stage NE-SW-trending fault zones gets obscured at sites of intense interaction with the early formed NW-SE-striking fault zones. In such areas, E of DBs consistently increases with proximity to the NW-SE-oriented fault zones revealing that the structural position of pre-existing fault zones mostly controls the geomechanical property distribution at sites of intense fault interaction. Similarly, we observe a direct relationship between Vp and grain size of the host rocks across the late-stage faults where there is relatively less fault interaction. However, this relationship does not exist within zones of intense fault interaction. As DBs are prevalent mostly in coarse-grained rocks, Vp in coarse-grained host rocks increases with proximity to the constrained fault zones. An integrated approach adopted in our study is crucial in delineating the effects of fault interaction on the distribution of geomechanical properties.

Effects of seepage pressure on the mechanical behaviors and microstructure of sandstone

Surrounding rocks of underground engineering are subjected to long-term seepage pressure, which can deteriorate the mechanical properties and cause serious disasters. In order to understand the impact of seepage pressure on the mechanical property of sandstone, uniaxial compression tests, P-wave velocity measurements, and nuclear magnetic resonance (NMR) tests were conducted on saturated sandstone samples with varied seepage pressures (i.e. 0 MPa, 3 MPa, 4 MPa, 5 MPa, 6 MPa, 7 MPa). The results demonstrate that the mechanical parameters (uniaxial compressive strength, peak strain, elastic modulus, and brittleness index), total energy, elastic strain energy, as well as elastic strain energy ratio, decrease with increasing seepage pressure, while the dissipation energy and dissipation energy ratio increase. Moreover, as seepage pressure increases, the micro-pores gradually transform into meso-pores and macro-pores. This increases the cumulative porosity of sandstone and decreases P-wave velocity. The numerical results indicate that as seepage pressure rises, the number of tensile cracks increases progressively, the angle range of microcracks is basically from 50°-120° to 80°-100°, and as a result, the failure mode transforms to the tensile-shear mixed failure mode. Finally, the effects of seepage pressure on mechanical properties were discussed. The results show that decrease in the effective stress and cohesion under the action of seepage pressure could lead to deterioration of strength behaviors of sandstone.

Effects of partial saturation on the liquefaction resistance of sand and silty sand from Christchurch

The liquefaction resistance of partially saturated soil was experimentally investigated for one clean sand and one silty sand collected from a site in Christchurch, in an area severely affected by liquefaction in the 2010–2011 Canterbury earthquakes. A series of cyclic undrained tests were performed on fully and partially saturated sand and silty sand specimens, in conjunction with evaluation of saturation conditions in situ based on comprehensive field measurements of P-wave velocity (Vp) in Christchurch deposits. The Skempton’s B-value and P-wave velocity were comparatively used as measures for partial saturation in the laboratory. B-value - Vp relationships from the test results indicate that Vp steadily increases with the B-value until a threshold B-value is reached beyond which Vp remains unchanged at values indicating full saturation, i.e. Vp ;≥ ;1600 ;m/s. In general, the liquefaction resistance of tested sand and silty sand increases with a decrease in the B-value or Vp, i.e. with a reduction in the degree of saturation. Furthermore, test results suggest existence of threshold B-values and Vp for tested soils beyond which no significant increase in the liquefaction resistance was observed. This threshold B-values and Vp were found to be dependent on soil type and applied confining stress. The effects of partial saturation on liquefaction strength are different for the sand and silty sand when using Vp as a measure for the degree of saturation. While a gradual rate of increase in liquefaction strength with decreasing Vp is observed for the tested sand, the liquefaction strength of silty sand shows similar gradual increase with a decrease in Vp up to about 800 ;m/s, which is then followed by an abrupt increase in the liquefaction strength for Vp ;< ;800 ;m/s. Generally good agreement between liquefaction strength of tested soils and published data was observed, with a clear distinctive feature in the behaviour of the silty sand as compared to clean sands.

Evaluation of EDZ characterization technology for URL based on field investigation at BET

Detailed characterization of the Excavation Damage Zone (EDZ) is an important issue for the high-level radioactive waste (HLW) geological disposal engineering, which is closely related to the long-term safety of the HLW repository. With drill-blast experiment in Beishan Exploration Tunnel (BET, a platform for the field experiment and validation of potential technologies used for China’s HLW disposal engineering in crystalline rock), systematic field tests were carried out to evaluate the suitability of techniques in characterizing EDZ, and to study EDZ characteristic in crystalline tunnel. A series of investigation technologies were used to delineate EDZ and to examine the process of its formation, which includes: p-wave velocity measurement, ground penetrating radar (GPR), acoustic emission (AE) monitoring and borehole televiewer. P-wave velocity measurement along borehole reveal that wave velocity decreased significantly (more than 10%) in 30 cm to 50 cm from the excavation surface, and this zone is defined as EDZ. Clear images of borehole televiewer were obtained, which is helpful for EDZ delineation. GPR and borehole radar were also used for EDZ delineation. Strong reflection signals were detected in the range of 10 cm to 25 cm by GPR with high frequency antenna, indicating significant changes of physical properties of rock-mass in the zone, which is defined as Highly Damaged Zone (HDZ). Moreover, the formation process of EDZ induced by blast in crystalline rock tunnel was analyzed using AE monitoring. Damage process of surrounding rock was divided into blasting damage stage and progressive damage stage. AE events location show the induced cracking activities were clustering within 30 cm to 100 cm from excavation surface. With the experience from field investigation and result analysis, the feasibility and suitability of the techniques for EDZ characterization is evaluated. Finally a specialized EDZ characterization technique system is proposed for underground research laboratory construction in crystalline host rock in Beishan area.

Mechanical weakening behavior and energy evolution characteristics of shale with different bedding angles after microwave irradiation

Microwave irradiation, as a novel, clean, and efficient rock-breaking technology, can be employed in shale gas exploitation to address the challenges of low fracture efficiency and environmental pollution. Therefore, it is essential to investigate the physico-mechanical properties of shale under microwave irradiation. Firstly, samples with different bedding angles (0°, 30°, 60°, and 90°) were subjected to microwave irradiation at a power of 1 kW and frequency of 2.45 GHz for durations of 0, 20, 40, 60, and 80 s. Additionally, measurements of mass, P-wave velocity, and porosity were taken before and after microwave irradiation, followed by uniaxial compression testing using acoustic emission (AE). The results demonstrated that with increasing microwave irradiation duration, the P-wave velocity and mass of shale gradually decreased, exhibiting anisotropic behavior for the P-wave velocity. The highest degree of weakening (24.75%) was observed for shale at 0°, while the lowest (7.03%) occurred at 90°. Both uniaxial compressive strength and elastic modulus weakened gradually. Notably, USC displayed more significant weakening at 30° and 60°. The energy evolution pattern revealed increasing dissipated energy and decreasing brittleness index, enhancing anisotropy with duration, particularly at 60°. Moreover, AE captured the transition of failure mode from tensile splitting to tensile-shear failure. The results indicated the significant and anisotropic weakening effect of microwave irradiation on shale. The increase in porosity and permeability also provided preliminary evidence for the enhanced permeability ability of shale, resulting in reduced hydraulic fracturing pressures and decreased environmental pollution. Furthermore, the changes of most parameters revealed threshold effect of microwave irradiation duration, which can be classified as weak or strong depending on whether it occurs before or after 40 s. Therefore, attention should be paid to the threshold conditions of microwave irradiation duration to achieve efficient extraction. This research provided initial theoretical support for microwave-assisted shale gas exploitation.

Effects of Water—Binder Ratio on Strength and Seismic Behavior of Stabilized Soil from Kongshavn, Port of Oslo

In many civil engineering problems, soil is stabilized by a combination of binders and water. The success of stabilization is evaluated using seismic tests with measured P-wave velocities. Optimization of process, laboratory testing and data modelling are essential to reduce the costs of the industrial projects. This paper reports the optimized workflow of soil stabilization through evaluated effects from the two factors controlling the development of strength: (1) the ratio between water and binder; (2) the proportions of different binders (cement/slag) were changed experimentally in a mixture of samples to evaluate the strength of soil. The experimental results show an optimal combination of 30% cement and 70% slag with a binder content of 120 kg/m3 and a maximum water binder ratio (w/b) of 5. Such proportions of mixture demonstrated effective soil stabilization both on a pilot test scale and on full scale for industrial works. The correlation between the compressive strength and relative deformation of specimens revealed that strength has the highest values for w/b = 5 and the lowest for w/b = 7. In case of high water content in soil and wet samples, the condition of a w/b ≤ 5 will require a higher amount of binder.

Detection of deteriorations in cultural structures created by carving into low-strength rocks by the non-destructive test (NDT) method

In the cultural stone heritage, progressive deteriorations develop over time with the effect of atmospheric processes. These deteriorations can reach to a significant degree that threaten the integrity of the monuments built from weak-strength rocks. In this study, it is aimed to determine the deteriorations caused by atmospheric processes on the monument surface in cultural heritage works built by carving into low-strength pyroclastic rocks by non-destructive tests (NDTs). To this end, two historic structures in the Kilistra Ancient City of Konya (Turkey) were selected. The index, strength, mineralogical and petrographic properties of the rocks, in which the monuments were carved, were first investigated. Then, contour scaling, crack, efflorescence and biodeterioration types were determined on the facades of the monuments. Later, NDT deterioration change maps were prepared based on the data obtained by performing the relative humidity, Schmidt hammer rebound (SHR), and P-wave velocity (Vp) measurements on the facades of the monuments. In the deterioration maps, it was determined that the SHR and Vp values of the rock were low in the capillary, infiltration, and crack zones with water penetration in the monuments built on low-strength pyroclastic rocks. However, deterioration was less in the regions with more limited water access according to zones.

Shear-wave velocity estimation based on rock physics modelling of a limestone gas reservoir in the Pannonian Basin

In the Pannonian Basin, especially in Croatia, there are a small number of wells for which acquired shear-wave (S-wave) velocities have been obtained. Often, quantitative interpretation relies only on compressional-wave (P-wave) velocity data, and S-wave velocity must be modelled. S-wave velocity estimation in combination with other petrophysical data is essential for detailed reservoir characterization. P- and S-wave velocities enable seismic modelling of different saturation states in a reservoir. This paper demonstrates a workflow for S-wave velocity estimation where S-wave velocity data are absent in the gas field and neighbouring fields with the same lithology, based on the Kuster–Toksöz and Xu–Payne models applied to the Pannonian Basin limestone reservoir. The results are calibrated with the P-wave velocity obtained from borehole data and the vertical seismic profile (VSP) S-wave interval velocity. Although rock physics models are idealized analogues of real rocks, a very good correlation was obtained between the modelled and measured P-wave velocity, as well as between the modelled S-wave velocity and the VSP interval velocity. The study also illustrates the problem of defining the pore aspect ratio in zones of increasing shale content. Due to the limited research on the limestones of the Pannonian Basin, these results enable a better understanding of the seismic parameters of the Pannonian Basin limestones. The results indicate that the proposed workflow gives an adequate estimation of S-wave velocities.

Improvement of cyclic liquefaction resistance induced by partial saturation: An interpretation using wave-based approaches

Methods to estimate the liquefaction resistance in sandy soils have been proposed in the literature, involving correlations between cyclic resistance ratio and seismic wave velocities measured by in situ and laboratory tests. However, the applicability of these correlations has not been validated in sands improved with the induction of partial saturation. This study addresses the assessment of cyclic liquefaction resistance improvement induced by partial saturation using wave-based approaches. For this purpose, an experimental plan comprising cyclic triaxial and bender elements was carried out in a Portuguese natural sand. The studied soil is TP-Lisbon sand, a historically liquefiable soil collected in the ‘Terreiro do Paço’ site — next to the Tagus River in the centre of Lisbon. Notwithstanding, the results presented herein are compared against other liquefiable sands from Japan. The induced partial saturation was controlled by P-wave velocity measurements, which were contrasted against theoretical predictions performed in the light of Biot’s theory. The results confirmed the increase in liquefaction resistance by inducing partial saturation. Experimental results revealed that P-wave velocity provides a reliable prediction of liquefaction resistance in partially saturated conditions rather than S-wave velocity. The suitableness of these approaches was attributed to the presence of occluded air bubbles in partially saturated soils, which can be detected by the P-wave and not by the S-wave.

Laboratory Experiments on Soil Stabilization to Enhance Strength Parameters for Road Pavement

AbstractClay soils can cause significant distress in road construction due to their low strength. Stabilizing such soil improve with binder agents prior to the geotechnical works can significantly its performance and ensure safety and stability of roads while exploitation. This research envisaged the use of five different binders (lime, energy fly ash, bio fly ash, slag, cement) as an additive stabilizing agents to improve the strength parameters of soil as required in engineering industry standards. The variations of strength was assessed using measurements of P-wave velocity of the elastic waves propagating through soil specimens stabilized by different combination of binders. Measurements were performed on 28thday of soil treatment. The best effects of added binders were noted in the following combinations: cement / energy fly ash / bio fly ash (P-waves &gt;3100 m/s), followed by combination lime / energy fly ash / GGBFS (P-waves &gt;2800 m/s) and cement / lime / energy fly ash (P-waves &gt;2700 m/s). Adding lime is effective due to its fixation and chemical bond with particles. The study contributes to the industrial tests on soil strength for constructing roadbed.

LITHOLOGY DISCRIMINATION USING SHEAR AND COMPRESSIONAL WAVES IN UYO AND ITS ENVIRON, SOUTHEASTERN NIGERIA

Compressional and shear wave velocities reveal vital information about the physical properties of rocks and their lithology contrast. In this study, we have applied the seismic refraction method to investigate the dynamic elastic modulus of subsurface layers in our study area. Electromagnetic geophones were firmly coupled to the ground to transform generated seismic energy at the source to electrical voltage which is a function of velocity. A total of twenty-seven P-wave refraction shootings were performed and measured P-wave velocity ranged from 218.00 ms-1 to 749.30 ms-1. The S-wave velocity obtained in the field ranged from 98.30 ms-1 to 340.70 ms-1. These values indicate that both layers 1 and 2 are weathered zones characterized by fine and lateritic sands which are capable of withstanding little or no mechanically related load or stress / pressure applied on the surface. In terms of Vp-Vs ratio, the first layer indicated a range of 2.2071 to 2.2183 with a mean value of 2.2108 for the area while the second layer produced a Vp-Vs ratio ranging from 2.1993 to 2.2055 with a mean value of 2.2023. The near constant Vp-Vs ratio of 2.2 obtained in the study shows a similarity in the formation grains in the shallow layers penetrated by the seismic waves.

Evaluating uncertainty in empirically derived unconfined compressive strength (UCS) estimates and implications for drilling applications; a case study from the Cooper Basin

Accurate estimates of Unconfined Compressive Strength (UCS) are essential for a range of subsurface applications, including drilling wells for subsurface fluid extraction or injection. However, measuring UCS of subsurface rock samples through laboratory-based uniaxial and triaxial testing is time consuming, expensive, and potentially subject to individual sample variance. P-wave velocity logs are routinely obtained in petroleum basins and are commonly used to estimate UCS using empirically-derived correlations. In this study we analysed P-wave velocity data from 43 wells in the Cooper Basin, Australia and created mean ranges of expected UCS through the Tirrawarra, Toolachee, Patchawarra, Murteree Shale and Epsilon formations, using literature-derived P-wave velocity-UCS correlations, and also estimated UCS based on selected available laboratory-determined P-wave velocity measurements. These two suites of P-wave velocity-derived estimates of UCS are then compared to available results from laboratory uniaxial testing of core samples. Our analysis indicates that P-wave velocities, and subsequent derived estimates of UCS from P-wave wireline logs, are typically lower than those from laboratory-analysed samples. This may reflect a sampling bias towards the selection of strong rocks for laboratory UCS testing or lower estimates of UCS due to wellbore damage and pore space content (air or liquids) from P-wave log derived velocities. However, the laboratory-derived P-wave velocities generally agree with laboratory-derived UCS data from the Patchawarra, Murteree Shale and Epsilon formations when using published empirical velocity-strength correlations. A retrospective case study presents the impacts of different UCS estimates on mud weight required to produce observed borehole breakouts in the Epsilon Formation. Breakouts were observed 90 degrees to the orientation of SHmax (117° N) with a mean width of 51°, the breakouts were produced during drilling using a 9.7 ppg mud weight. The back-calculated estimate of UCS from the observed breakout widths at 9.7 ppg is 154 MPa. This sits between the UCS estimate of 171 MPa from uniaxial testing and 145 MPa from the mean of laboratory sample-derived P-wave velocities. Estimates of UCS from empirical correlations based solely on p-wave log-derived are far lower and vary between 87.3 and 114.4 MPa.

Strain Partitioning and Frictional Behavior of Opalinus Clay During Fault Reactivation

The Opalinus Clay (OPA) formation is considered a suitable host rock candidate for nuclear waste storage. However, the sealing integrity and long-term safety of OPA are potentially compromised by pre-existing natural or artificially induced faults. Therefore, characterizing the mechanical behavior and microscale deformation mechanisms of faults and the surrounding rock is relevant for predicting repository damage evolution. In this study, we performed triaxial tests using saw-cut samples of the shaly and sandy facies of OPA to investigate the influence of pressure and mineral composition on the deformation behavior during fault reactivation. Dried samples were hydrostatically pre-compacted at 50 MPa and then deformed at constant strain rate, drained conditions and confining pressures (pc) of 5–35 MPa. Mechanical data from triaxial tests was complemented by local strain measurements to determine the relative contribution of bulk deformation and fault slip, as well as by acoustic emission (AE) monitoring, and elastic P-wave velocity measurements using ultrasonic transmissions. With increasing pc, we observe a transition from brittle deformation behavior with highly localized fault slip to semi-brittle behavior characterized by non-linear strain hardening with increasing delocalization of deformation. We find that brittle localization behavior is limited by pc at which fault strength exceeds matrix yield strength. AEs were only detected in tests performed on sandy facies samples, and activity decreased with increasing pc. Microstructural analysis of deformed samples revealed a positive correlation between increasing pc and gouge layer thickness. This goes along with a change from brittle fragmentation and frictional sliding to the development of shear zones with a higher contribution of cataclastic and granular flow. Friction coefficient at fault reactivation is only slightly higher for the sandy (µ ~ 0.48) compared to the shaly facies (µ ~ 0.4). Slide-hold-slide tests performed after ~ 6 mm axial shortening suggest stable creeping and long-term weakness of faults at the applied conditions. Our results demonstrate that the mode of fault reactivation highly depends on the present stress field and burial history.

Experimental research on propagation and attenuation of ultrasonic waves in water-bearing coal

Ultrasound has a very broad application prospect in monitoring the wetting range of coal seam water injection. In order to explore the evolution of P-wave velocity with parameters such as moisture content and pore size of coal during coal seam water injection, ultrasonic P-wave velocity measurement experiments are carried out for coal samples with different moisture contents, densities and pore sizes in this paper. Through analysis and discussion of the results, the following conclusions are drawn. (1) The P-wave velocity increases gradually with the increase of the coal moisture content. In the three coal samples prepared from three different meshes of pulverized coal, the P-wave velocities show an increasing trend of logarithmic function, linear function and exponential function, respectively. (2) With the decrease of the coal density, the P-wave velocity of the water-bearing coal sample first decreases, then increases, and then decreases again. When the density of the coal sample decreases to a certain extent, the moisture has a great influence on the P-wave velocity. When the density further decreases, the influence of density on the P-wave velocity plays a dominant role again. (3) With the increase of the pore size, the P-wave velocity of the water-bearing coal sample gradually decreases. The P-wave velocities of water-bearing coal with different pore sizes have different variation ranges, and their average values are quite different. Compared with the moisture content of the coal sample, the pore size has a greater impact on the P-wave velocity.